Study On Influence Of Foundation Pit Excavation On Deformation Of Existing Underlying Tunnel And Its Protection Measures

With the continuous development of urban underground space,there are also more and more subway tunnels,at the same time,the excavation of foundation pit is one of the most extensive projects,as a result,there are more and more excavation projects close to existing tunnels,which will have a great adverse impact on the tunnel.If there is a lack of reasonable research and analysis before the foundation pit construction and no reasonable measures are taken during the construction period,the subway deformation may be too large,and there will be wrong or rupture of tube segments,water leakage of joints and other situations,and even threaten the safety of subway operation and normal use,causing serious economic losses and huge social impact,therefore,it is very important to analyze the deformation influence of the existing tunnel caused by excavation unloading and how to control the deformation of the existing tunnel.Based on the project of an existing double-track tunnel across a foundation pit in soft soil area of Shanghai,this paper analyzes and studies the influence of foundation pit excavation unloading on the deformation of underlying tunnel and the effect of existing tunnel protection measures to control tunnel deformation by using theoretical analysis and numerical simulation methods.The specific research work is as follows:(1)In this paper,the vertical deformation mechanism of the existing tunnel caused by the unloading of foundation pit construction is briefly analyzed,and then the two-stage stress method is used for theoretical calculation of the case of this project,the first phase is caused Mindlin solution is used to calculate the bottom of the excavation unloading the vertical additional stress of the tunnel,the second stage,Winkler and Pasternak foundation beam models are used to calculate the vertical deformation of the tunnel by the finite difference method,the vertical deformation curve of tunnel under two kinds of foundation beam models is studied and analyzed(2)The ABAQUS finite element software was used to carry out modeling calculation of this project,and compared with the theoretical calculation results,it was found that the trend of curves calculated by numerical simulation was generally consistent with the theoretical calculation.In addition,the numerical simulation method is used to study the influence of foundation pit excavation construction on the deformation of the underground tunnel,mainly analyzing the influence factors such as the space effect of foundation pit excavation,the depth of foundation pit excavation and the plane size,the main analysis discusses the spatial effect of different block and sequence,as well as the law of the influence of different foundation pit depth and plane size on the deformation of the underlying tunnel during foundation pit excavation.(3)Numerical simulation research on the protection measures of existing tunnels was carried out,the effects and laws of different protection measures such as the partition wall,the depth of soil grouting,pile loading,the setting of uplift piles and the bottom plate to control the deformation of the tunnel were analyzed separately.Combining measures to study the effect of a variety of protective measures to jointly control the deformation of the tunnel,and put forward some suggestions...